docs: fix broken Markdown link (issue #10018)

[d-mahi14]

Description

A broken link was detected in the automated Markdown links workflow for the Schubert and Gertz (2018) reference. The standard DOI redirector was returning a 404.

This PR updates the link to point directly to the ACM Digital Library landing page, which resolves correctly and satisfies the link-checking workflow.

Related Issues

resolves #10019

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